Laser-transferred ibc solar cells

ABSTRACT

A laser processing system can be utilized to produce high-performance interdigitated back contact (IBC) solar cells. The laser processing system can be utilized to ablate, transfer material, and/or laser-dope or laser fire contacts. Laser ablation can be utilized to remove and pattern openings in a passivated or emitter layer. Laser transferring may then be utilized to transfer dopant and/or contact materials to the patterned openings, thereby forming an interdigitated finger pattern. The laser processing system may also be utilized to plate a conductive material on top of the transferred dopant or contact materials.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/024,784, filed on Jul. 15, 2014, which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to interdigitated back contact (IBC) solar cells.More particularly, to systems and methods for fabricating IBCs.

BACKGROUND OF THE INVENTION

A desirable solar cell geometry referred to as an interdigitated backcontact (IBC) cell comprises a semiconductor wafer and alternating lines(interdigitated stripes) of regions with p-type and n-type doping. Thiscell geometry has the advantage of eliminating shading losses altogetherby putting both contacts on one side of the wafer. Further, contacts areeasier to interconnect with both contacts on the rear. A laser-transferprocess developed by Mei et al. at Xerox Palo Alto Research Center(PARC) is discussed in U.S. Pat. No. 5,871,826 entitled “Proximity LaserDoping Technique for Electronic Materials” was issued on Feb. 16, 1999.The patent describes a method of altering the electrical characteristicsof a material through a laser ablation process, which can achieve highdoping levels and shallow junctions at low temperatures. The inventionutilizes a rapid interaction between a laser and a non-transparent thinsource film deposited on a transparent plate (typically glass orquartz), which is placed in close proximity (typically about severalmicrons) to a substrate. A process is described where a laser, such as aYAG laser, with a pulse duration of typically about 50 ns can be used toform a semiconductor junction with a depth of about 0.1 μm by using from16 to 400 shots at a laser energy density ranging from 150 to 450mJ/cm². This proximity laser ablation technique can be used to depositthin films over a large area substrate at low temperature, and one mayalso use a mask to block off the laser energy in areas where depositionis not desirable.

More recently, Roder et al. (Proc. of the 35^(th) IEEE PhotovoltaicSpecialists Conference, pp. 3597-3599, (2010)) at the University ofStuttgart used a similar process, which they called “Laser TransferredContacts” (LTC) or “Laser Induced Forward Transfer” (LIFT) to lasertransfer a thin layer of Ni through a silicon nitride antireflectioncoating to form a contact to a laser-doped selective emitter region. Thelaser-transferred Ni contact was then electroplated with 3 μm of Ni, andthen plating continued with Cu to increase the conductivity of thefingers. With this technique, a 17.4% efficient silicon solar cell wasfabricated with 30 μm wide finger contacts.

Scientists at the University of Stuttgart (Hoffmann et al., Proc. of the38^(th) IEEE Photovoltaic Specialists Conference, pp. 1059-1062 (2012))also demonstrated a self-doping laser transferred contact process whereSb contacts were laser transferred through a silicon nitrideantireflection coating to provide a self-aligned n-type selectiveemitter and simultaneously formed the contacts to the front side of thesolar cell. The antimony doped contacts were used as a seed layer forsubsequent nickel and copper plating, and were able to produce a fineline front metallization with a finger width of 20 μm and contactresistivity as low as 30 μΩ-cm² on a 110 Ω/sq. emitter. A green (532 nm)Nd:YAG laser was used with a pulse duration of 30 ns, as well as a greenNd:YVO₄ laser with a pulse duration of 6 ns, in conjunction with anoptical system that shaped the beam into a line focus in order tominimize defect creation during the recrystallization of the Si. Contactlines with widths of ˜7 μm were obtained, and Ni/Cu electroplating wasused to increase the conductivity of the fingers. Solar cells withefficiencies as high as 17.5% were demonstrated.

Presently, Si solar cells with the highest efficiency are those based oncombining an interdigitated all back contact structure with siliconheterojunction contacts. Panasonic recently reported obtaining a recordconversion efficiency of 25.6% with such a device structure (Masuko etal., 40^(th) IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014,Denver, Colo.). At the same conference, Sharp reported obtaining anefficiency of 25.1% with a similar device structure (Nakamura et al.,40^(th) IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014,Denver, Colo.), and SunPower obtained an efficiency of 25.0% with aninterdigitated back contact (IBC) silicon solar cell made usingconventional diffusion processes (Smith et al., 40^(th) IEEEPhotovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.).While the processing of these high efficiency IBC solar cells were notdiscussed in any detail, the manufacturing costs are likely to berelatively high since the processing in each case appears to be somewhatcomplicated with various masking and vacuum processing steps required.

Laser processing has also been used to fabricate relatively highefficiency silicon solar cells. Benick et al. (40^(th) IEEE PhotovoltaicSpecialists Conference, Jun. 8-13, 2014, Denver, Colo.) obtainedconversion efficiencies as high as 23.2% in a PERL (Passivated Emitterand Rear Locally-diffused) solar cell structure by laser dopinglocalized rear contacts. In this process, a rear surface passivationlayer consisting of a phosphorus-doped amorphous silicon carbide(a-SiC_(x):P) as the doping source was utilized. The front surfaceemitter was formed by boron diffusion, and the front side contacts wereformed using photolithography and evaporating a seed layer of Ti/Pd/Agand then plating with Ag.

Dahlinger et al. (Energy Procedia 38, 250-253 (2013)) used laser dopingto fabricate an interdigitated back contact silicon solar cell with anefficiency of 22.0%. A frequency doubled (532 nm) Nd:YAG-laser with aline shaped beam (<10 μm wide) and a pulse duration of ˜50 ns was used.A thin boron precursor layer was sputtered on the rear side of thewafer, and then a laser doped p⁺ emitter pattern was formed. After wetchemical cleaning, a lightly phosphorus doped region was formed on bothsides of the wafer using POCl₃ furnace diffusion. Another laser dopingprocess (utilizing the phosphosilicate glass grown during the POCl₃diffusion as the doping source) was used to create the n⁺ pattern on therear side of the wafer. Thermal oxidation was used to drive in thediffused regions and to passivate the surface, and a plasma-enhancedchemical vapor deposited (PECVD) silicon nitride was used to form ananti-reflection coating on the front side and also an infraredreflection coating on the rear side of the wafer.

Hofmann et al. (Progress in Photovoltaics: Research and Applications; 16509-518 (2008)) fabricated 21.7% Si solar cells used laser firing of Althrough a rear surface passivation stack of amorphous silicon and PECVDsilicon oxide to form localized p⁺ contacts. The front surface emitterwas formed using phosphorus diffusion at elevated temperatures, and athermally oxidized anti-reflection coating was used that also served asfront passivation layer. The front contacts were formed by evaporating aTiPdAg finger pattern.

While it is clear that laser processing can be used to make relativelyefficient silicon solar cells, the processing used to date is somewhatcomplicated and in all cases uses some high temperature processing alongwith vacuum processing. High temperature processing can create defectsin the silicon wafer, which can limit solar cell performance, and vacuumprocessing requires relatively expensive vacuum equipment, which canlimit throughput and add to the manufacturing costs.

SUMMARY OF INVENTION

In one embodiment, interdigitated back contacts are formed in apassivated solar substrate by laser transferring both p+ and n+ fingerpattern seed layers through the dielectric passivation using a lasertransfer process with a narrow line-shaped laser beam. The seed layersmay be plated with a conductive metal.

In another embodiment, a laser transfer process is used on awell-passivated solar substrate to fire p+ and n+ point contacts throughthe dielectric passivation. Another laser transfer process is then usedto deposit an interdigitated finger pattern of an appropriate metal ontop of the dielectric passivation and over the appropriate pointcontacts using a narrow line-shaped laser beam.

In yet another embodiment, a finger pattern is formed on awell-passivated solar substrate where most of the rear surface containsa tunnel oxide emitter by laser ablating the tunnel oxide emitter usingline-shaped laser beams and laser transferring a base contact fingerpattern.

In other embodiments, the laser transfer system may utilize either anarrow line-shaped laser beam or a small Gaussian laser beam, either ofwhich can be temporally shaped, to either ablate, transfer a dopant,metal or other material; or laser-dope or laser fire localized p+ or n+contacts.

The foregoing has outlined rather broadly various features of thepresent disclosure in order that the detailed description that followsmay be better understood. Additional features and advantages of thedisclosure will be described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1 is an illustrative embodiment of lines (e.g. Al and Sb) that arelaser transferred through the passivation layers as interdigitated p⁺and n⁺ finger patterns, respectively;

FIG. 2 is an illustrative embodiment of an interdigitated finger patternformed utilizing seed layers;

FIG. 3 is an illustrative embodiment of n⁺ and p⁺ point contacts madeusing a Gaussian laser beam to laser transfer small spots of n+ and p+materials (e.g. Sb and Al), respectively, through the passivationlayers;

FIG. 4 is an illustrative embodiment showing an interdigitated seedlayer (e.g. Ni) that is laser transferred on top of the passivationlayers and on top of the n⁺ and p⁺ point contacts;

FIG. 5 is an illustrative embodiment of a device structure with a tunneloxide emitter and where a laser beam is used to ablate a line region ofa substrate and then to laser transfer a base dopant in a centralregion; and

FIG. 6 is an illustrative embodiment of a schematic of a laser transfersystem that utilizes a scanning line-shaped laser beam.

DETAILED DESCRIPTION

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing particularimplementations of the disclosure and are not intended to be limitingthereto. While most of the terms used herein will be recognizable tothose of ordinary skill in the art, it should be understood that whennot explicitly defined, terms should be interpreted as adopting ameaning presently accepted by those of ordinary skill in the art.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only,and are not restrictive of the invention, as claimed. In thisapplication, the use of the singular includes the plural, the word “a”or “an” means “at least one”, and the use of “or” means “and/or”, unlessspecifically stated otherwise. Furthermore, the use of the term“including”, as well as other forms, such as “includes” and “included”,is not limiting. Also, terms such as “element” or “component” encompassboth elements or components comprising one unit and elements orcomponents that comprise more than one unit unless specifically statedotherwise.

Systems and methods for producing high-performance interdigitated backcontact (IBC) solar cells that are fabricated at low temperatures withlow manufacturing costs using a laser-transfer process are discussedherein. The laser-transfer process may utilize spatially and/ortemporally shaped laser beams. In some embodiments, the systems ormethods discussed herein may have the following elements: (1) supply ofdopants by a laser transfer process; (2) one or more dopants supplied bylaser transfer process to avoid heating of the wafers to perform dopantdiffusion; and/or (3) deposition in the processing of a back contact(IBC) cell. The use of line beams is a particularly attractive way tomake an IBC cell since the electrodes of the IBC are thin lines, andthus interdigitated fingers can be patterned with a reduced number oflaser pulse exposures. As a nonlimiting example, a Gaussian beam from ahigh power laser could be transformed into a long, narrow line-shapedbeam using the appropriate optics. If the line beam is 1 cm long and 8microns wide, then 15 pulses could be used to laser transfer conductivematerial for a single finger that is 15 cm long in an interdigitatedfinger pattern. If one used Gaussian beams 100 microns in diameter andthey overlap by 10%, then it would take 1875 pulses to create a finger15 cm long. Thus, the dramatic reduction (>99%) in pulses in readilyapparent from the examples above. In some embodiments, the combinationof (1)-(3) above may be utilized with line and/or temporal shaping.

The term “solar substrate” may be used herein to describe a siliconwafer that has been partially processed and will become a functionalsolar cell when all processing steps are completed. It should beunderstood that a solar substrate may sometimes be referred to as asolar cell below despite being in an intermediate state prior to theformation of a functional solar cell.

In some embodiments, the laser beam can be spatially shaped into anarrow line-shaped laser beam or into an array of very small diameterGaussian laser beams (e.g. <20 μm or <10 μm). In some embodiments, thelaser transfer process is a low heat process where the majority of thewafer remains at room temperature, and the laser energy is adjusted sothat there is sufficient energy to transfer material to the wafer. Insome embodiments, the low heat process may allow for localized heatingof the wafer that is limited to temperatures well below the meltingpoint of silicon (or equal to or less than 1,414° C.). As discussedabove, work performed at the University of Stuttgart indicates thatline-shaped laser beams with widths <10 μm exhibit little laser-induceddamage, while conventional circular Gaussian laser beams (e.g. withdiameters of ˜30-130 μm) exhibit microcracks and dislocations. In someembodiments, the improved laser process discussed herein may utilizesmall diameter (<20 μm or <10 μm) Gaussian laser beams, which are alsoless likely to exhibit extended defects such as microcracks anddislocations due to the fact that only a very small region of Si ismelted and recrystallized. In some embodiments, a pulsed line-shapedlaser beams may be capable of processing about 150 meters or greater offinger length per second or about 1 silicon wafer or greater per second,which is approximately 100× greater processing throughput than withpulsed Gaussian laser beams.

In some embodiment, laser beams may be temporally shaped by scanning thelaser system along the silicon wafer substrate and pulsing the laser adesired number of times to form a desired pattern, such as a fingerpattern for interdigitated back contacts. The temporal pulse shape canbe selected for the purposes of laser transfer of material, laserablation or disruption of dielectric passivation layers, laser meltingof selected localized regions of the Si wafer, laser doping of themelted Si regions with the appropriate dopant atoms, laser firing ofcontacting metals through the dielectric passivation layers and laserannealing of the localized treated regions on the Si wafer. Generally,laser transfer of material requires relatively short pulses (few ns tofew tens of ns) while laser annealing requires relatively long pulses(0.1 μs to several μs). The pulse duration for laser doping will dependon the dopant depth required and can vary from tens of ns to hundreds ofns. As a nonlimiting example, a laser process which combines lasertransfer, disruption of the dielectric passivation, melting, doping andannealing of the Si in a localized region might employ a line-shapedbeam (e.g. 8 μm wide and 1 cm long) with the following temporallyshaping: the pulse starts with an energy density of ˜1 J/cm² overseveral ns to transfer the dopant material (e.g. Al) to the substrate(e.g. Si surface) and disrupt the dielectric passivation (e.g. 5 nm ofALD Al₂O₃/90 nm of PECVD SiO_(x) on the rear surface); the energydensity then falls to ˜0.5 J/cm² over ˜50 ns to locally melt thesubstrate surface and diffuse in the dopant; and then the pulse energydensity decreases from 0.5 to 0.1 J/cm² over ˜500 ns to anneal thelocalized region of substrate surface.

In a first embodiment, a passivated solar cell provides interdigitatedback contacts that are formed by laser transferring both p⁺ and n⁺finger pattern seed layers through the dielectric passivation using alaser transfer process with a narrow line-shaped laser beam, and thenplating the seed layers with a conductive metal. For example, in someembodiments, the line-shaped laser beam may be <20 μm or <10 μm. FIG. 1is an illustrative embodiment of lines 10, 20 that are laser fired orlaser transferred through the passivation layers 30, 50 asinterdigitated p⁺ and n⁺ finger patterns. As a nonlimiting example, thep⁺ finger pattern seed layer 10 could be formed by laser transferringdopant materials under conditions where the laser beam disrupts thedielectric passivation layers 30, 50, melts the substrate 40 (e.g. Si)and diffuses the dopant into the substrate. Further, the n⁺ fingerpattern seed layer 20 could be formed by laser transferring dopantmaterials under conditions where the laser beam disrupts the dielectricpassivation layers 30, 50, melts the substrate 40 (e.g. Si) and diffusesthe dopant into the substrate. In the case of the n⁺ finger pattern seedlayer 20, the laser conditions could be selected to promote disruptionof the dielectric passivation layers 30, 50 over a wider range so as tominimize current leakage from the inversion layer 60 created by thealuminum oxide 50 to the n⁺ fingers (note the gap between thepassivation layer 50 and n⁺ seed layer 20 shown in FIG. 1). Asnonlimiting examples, dopant materials may any suitable n- or p-typematerial, Al, Sb, Group III or V element, or the like. In the lasertransfer process, the dopant atom is introduced on a donor substrate 40containing or coated with a dopant material including the donor atom.The dopant material may be a pure form of the dopant, such as coatingsof the group III or group V atoms. Alternatively, the dopant materialmay be a compound containing the dopant, such as an oxide, nitride, orchalcogenide of the donor. The dopant material may also be composed of ahost material containing the dopant, such as amorphous silicon heavilydoped with the dopant. Concentration of the dopant in the host materialmay be greater than 0.5%, preferably greater than 2%. The spacing of theinterdigitated fingers in the laser-transferred line-contact IBC cellscan be made relatively small (e.g. 100-300 microns), so that the lateralresistance (electrical shading) in the device is small.

In some embodiments, interdigitated back contact (IBC) silicon solarcells that are fabricated at low temperatures with low manufacturingcosts using a laser-transfer process utilizing a narrow line-shapedlaser beam or small diameter Gaussian laser beams. In some embodiment,the interdigitated back contacts for a well-passivated solar cell areformed by laser transferring both p⁺ and n⁺ finger pattern seed layersthrough the dielectric passivation using a laser transfer process with anarrow line-shaped laser beam, and then plating the seed layers with aconductive metal. In some embodiments, a laser transfer process is usedon a well-passivated solar substrate to fire p⁺ and n⁺ point contactsthrough the dielectric passivation, and another laser transfer processis then used to deposit an interdigitated finger pattern of anappropriate metal on top of the dielectric passivation and over theappropriate point contacts using a narrow line-shaped laser beam. Insome embodiments, most of the rear surface of a well passivated solarcell contains a tunnel oxide emitter interspersed with parallel lines ofohmic base contacts in a finger pattern formed by laser ablating thetunnel oxide emitter and a base contact finger pattern laser transferredusing line-shaped laser beams. In various embodiments, the lasertransfer system, which utilizes either a narrow line-shaped laser beamor a small Gaussian laser beam, either of which can be temporallyshaped, can be utilized to ablate, transfer a dopant, metal or othermaterial, and/or laser-dope or laser fire localized p⁺ or n⁺ contacts.

In some embodiments, the Al and Sb lines 10, 20 are laser transferredthrough the passivation layers 30, 50 as interdigitated p⁺ and n⁺ fingerpatterns, respectively. In some embodiments, the Al₂O₃ passivation layer50 induces an inversion layer 60 which is in electrical contact with thelaser-transferred Al emitter lines 10. In some embodiments, thelaser-transferred Sb n⁺ lines 20 are deposited under conditions thatlocally disrupt the Al₂O₃ layer 50 to prevent shunting. Further, laserablation could also be used to locally remove the passivation layer 50before laser transferring the Sb lines 20. In some embodiments, the seedlayers could then be plated with a metal, such as Ni, Ti, or the like.Further, this may be optionally followed by plating with a moreconductive metal, such as Al, Ag, Cu, or the like, to form a highlyconductive interdigitated finger pattern.

FIG. 2 is an illustrative embodiment of an interdigitated finger pattern210, 220 formed utilizing seed layers, such as seed layers shown inFIG. 1. As shown, the finger pattern provides alternating approximatelyparallel horizontal lines of different materials separated by a smallgap, where one end of the horizontal lines of common materials areconnected by a vertical line of matching material. In some embodiments,the dielectric passivation layers could comprise ALD Al₂O₃ and PECVDSiO_(x):H on the rear surface. Subsequent to the laser transfer process,the solar cell can be annealed at moderate temperatures (200-450° C.) toimprove the electrical properties of the contacts by promoting silicideformation and inducing atomic hydrogen motion from the PECVD SiO_(x):Hinto the Si to passivate any laser-induced defects. In this example, Tibus bar seed layers 260 are laser transferred at a low laser power sothat they lie on top of the dielectric passivation layers. Thelaser-transferred Al 210 and Sb 220 lines, and the Ti bus bars 260 formthe seed layers for plating of a conductive metal, such as Ag, Al or Cu.

In another embodiment shown in FIG. 3, a laser transfer process is usedon a passivated solar substrate to fire p⁺ and n⁺ point contacts 310,320 through the dielectric passivation 330, and another laser transferprocess is then used to deposit an interdigitated finger pattern of anappropriate metal 370 on top of the dielectric passivation and over theappropriate point contacts using a narrow line-shaped laser beam. FIG. 3is an illustrative embodiment of n⁺ and p⁺ point contacts 310, 320 madeusing a Gaussian laser beam to laser transfer small spots of n+ and p+materials, respectively, through the passivation layers 330. Forexample, in some embodiments, the Gaussian laser beam may be smalldiameter beam, such as <20 μm or <10 μm. If small diameter beams areused, then the density of small spots should be high enough to assurethat the total contact resistance is less than 1 Ω-cm². In someembodiments, the total area of all small spots should be equal to orgreater than about 1% of the total rear surface area of the solarsubstrate. The same criterion also applies to line-shaped laser beams(i.e. the total area of all line-shaped contact regions should begreater than about 1% of the total rear surface area of the solarsubstrate). An interdigitated finger pattern is formed on thepassivation by using a line-shaped laser beam to deposit a seed layer,which may be plated with a highly conductive metal. FIG. 4 is anillustrative embodiment showing an interdigitated seed layer 470 (e.g.Ni) that is laser transferred on top of the passivation layers and ontop of the n⁺ and p⁺ point contacts 410, 420. In another embodiment, afinger pattern of n+ and p+ materials (e.g. Al and Sb) may be lasertransferred under conditions where they lie on top of the dielectricpassivation layers, and to then laser fire the p⁺ and n⁺ point contacts410, 420 through the dielectric passivation layers. Yet anotherembodiment is to laser transfer the n⁺ and p⁺ point contacts 410, 420before passivating the wafer, then laser transferring a Ni IBC pattern470 on top of the dielectric passivation layers and then laser firingthe Ni into the point contacts. However, this approach requires accuratealignment in order to laser fire the Ni into the point contacts.

As a nonlimiting example, the n⁺ and p⁺ point contacts are made using aGaussian laser beam to laser transfer small spots of Sb and Al,respectively, through the passivation layers. An interdigitated fingerpattern is formed on the passivation by using a line-shaped laser beamto deposit a seed layer of Ni, which is then plated with Cu. In thisexample, the rear surface passivation layers are a-Si:H/PECVD SiOx,which minimize shunting due to the lack of band bending. In thisexample, an interdigitated Ni seed layer is laser transferred on top ofthe passivation layers and on top of the n⁺ and p⁺ point contacts 410,420. An alternative approach is to laser transfer a finger pattern of Aland an interdigitated finger pattern of Sb, and then to laser fire thepoint contacts 410, 420. Another approach is to laser transfer the n⁺and p⁺ point contacts 410, 420 before passivating the wafer, then lasertransferring a Ni IBC pattern and then laser firing the Ni into thepoint contacts.

Another embodiment is a passivated solar cell where most of the rearsurface contains a tunnel oxide emitter interspersed with parallel linesof ohmic base contacts in a finger pattern formed by laser ablating thetunnel oxide emitter and laser transferring a base contact fingerpattern using line-shaped laser beams. While this embodiment discussestunnel oxide emitters, other embodiments may provide a diffused emitteror an amorphous silicon heterojunction emitter. FIG. 5 is anillustrative embodiment where a tunnel oxide layer 510 is firstdeposited on the rear surface by atomic layer deposition, and then athin layer of metal oxide(s) 520 (e.g. MoO_(x) and ZnO) are thendeposited. A line-shaped laser beam is used to ablate 530 a line region,and then a line 540 is laser transferred and doped in the central region(e.g. Sb). In some embodiments, it may be possible to laser transfer theSb under conditions that locally disrupt the tunnel oxide layers beyondthe doped region so that a separate laser ablation step is not required,thereby combining the above noted laser processing steps into one step.As shown in FIG. 5, the laser ablated 530 region is larger than thewidth of line 540 so that a separate laser ablation step is unnecessaryto provide separation between the line 540 metal oxide(s) 520. In thiscase, both the tunnel oxide layers and the Sb base contact may be platedwith conductive material (e.g. Ni/Cu) to increase the conductivity ofthe contacts.

For example, the tunnel oxide layers are first deposited on the rearsurface by ALD. A line-shaped laser beam ablates a line region, and thena line of Sb is laser transferred and doped in the central region.Another possibility is to laser transfer the Sb under conditions thatlocally disrupt the tunnel oxide layers. The structure is annealed &Ni/Cu is plated to the Sb and the tunnel oxide layers.

In the example shown, the tunnel oxide emitter is shown as a thin (1.4nm) SiO₂ layer coated with a high work function MoO_(x) layer (10 nmthick) and topped with a conductive ZnO layer (˜90 nm thick) to enhancethe reflection from a subsequently plated rear metal contact. TheMoO_(x) layer could be replaced with p-type a-Si:H or another high workfunction material. For a p-type wafer, a low work function layer wouldbe used. The tunnel oxide emitter would cover most of the rear surface(e.g. 95%) with the n⁺ base line contacts covering ˜5% (included anydisrupted or ablated regions). Thus, the n⁺ lines may be relatively thin(e.g. 8 microns wide) to minimize laser damage and spaced ˜200 micronsapart (the disrupted passivation regions might increase the effectiveline width to ˜10 microns). The front surface of the laser-transferredline-contact IBC solar cells could be passivated with a high qualityAl₂O₃ passivation layer to induce an accumulation layer (for p-typewafers) or an inversion layer (for n-type wafers). Since the spacing ofthe interdigitated fingers in the laser-transferred line-contact IBCcells is relatively small (e.g. 100-300 microns), the lateral resistance(electrical shading) in the device is small.

FIG. 6 is an illustrative embodiment of a schematic of a laser transfersystem that utilizes a scanning laser 600 that provides a line-shapedbeam 610. In yet another embodiment, a laser transfer system isprovided, which utilizes either a narrow line-shaped laser beam or anarray of small Gaussian laser beams (e.g. <20 μm or <10 μm), either ofwhich can be temporally and/or spatially shaped, to either ablate;transfer a dopant, metal, or other material; or laser-dope or laser firelocalized p⁺ or n⁺ contacts 620. This system uses thespatially/temporally shaped laser beams 610 to transfer and laser-fire(or laser dope) materials to be transferred 630, e.g. both p⁺ and n⁺dopants, through a high quality dielectric passivation layer on a solarsubstrate 640 to form a low-cost, high-performance, interdigitatedback-contact solar cell at low temperatures without the need for anyvacuum processing equipment. The laser transfer system may comprise alaser beam 610 with a temporally adjustable pulse that is optimized toproduce high quality localized emitters and base contacts 620. Forexample, as the system scans the substrate 640, the laser may beactivated at desired times when the laser is above an area to bepatterned to ablate, laser transfer materials, or laser dope thesubstrate in region-of-interest. The transparent transfer substrate 650(e.g. a thin glass plate) is positioned and/or held a fixed distancefrom the Si wafer 640 (e.g. 5-50 microns) by a spacer 660. The transfersubstrate 650 may provide regions deposited with one or more layers ofvarious materials 630 (e.g. metals (e.g Sb, Al), dopant materials (e.g.spin-on phosphorus or boron containing inks), materials to aid ablation,etc.) so that the laser can either transfer the materials to the wafer640 or ablate a dielectric surface on the Si wafer 640. In someembodiments, the transfer substrate 650 may not be coated with anymaterials or it may be removed, as it may not be necessary for processesinvolving ablation. By designing the system with interchangeable optics,one could laser transfer and dope p⁺ and n⁺ point contacts 620 and thenswitch to a low-power laser transfer of an interdigitated finger patternthat would lie on top of the dielectric passivation. The laser beam 610can be scanned across the transparent transfer substrate 650 and thesilicon wafer 640 to form the desired contact pattern on the surface ofthe wafer.

The laser transfer system can utilize multiple pulses in addition totemporally shaped pulses. For example, the first pulse could comprise afirst section of relatively high energy density (e.g. ˜1 j/cm²) over 10ns, and then a slowly decreasing section where the energy densitydecreases from 0.7 to 0.1 J/cm² over 500 ns. A second pulse to the samelocation might then be applied 10 μs is later (100 kHz repetition rate)with an energy density ramping up to ˜0.3 J/cm² over 10 ns, and thenslowly decreasing to 0.05 J/cm² over 500 ns to further anneal thetreated region. The wavelength of the laser beam can be in the IR (e.g.1064 nm) for most applications, but a laser beam operating in the green(532 nm) can also be used and will more effectively heat just the topfew μm of an exposed Si surface. The IR beam will initially heat the Siwafer to a depth of a few hundred μm, but as the laser rapidly heats upthe Si locally, the absorption coefficient in the IR increases rapidlyand the heating becomes localized near the surface region.

In some embodiments, the transfer substrate 650 of the laser transfersystem can be coated with multiple layers depending on the application.For example, the laser transfer substrate 650 may be first coated with athin easily evaporated material 630 (e.g. a-Si:H) to act as a releaselayer for a refractory material (e.g. Mo) or a transparent material(e.g. SiO₂) deposited on the a-Si:H. Another nonlimiting exampleinvolves first depositing a layer of Ni on the laser transfer substratefollowed by a layer of Sb so that the laser will transfer Sb for n⁺doping and Ni for a low-resistance nickel silicide contact.

For example, the laser transfer system may use a narrow line-shapedlaser beam and/or small Gaussian laser beams to transfer and laser-fire(or laser dope) both p⁺ and n⁺ dopants through a high quality dielectricpassivation layer to form a low-cost, high-performance, interdigitatedback-contact solar cell at low temperatures without the need for anyvacuum processing equipment. The laser transfer system comprises a laserbeam with a temporally adjustable pulse. The transparent transfersubstrate (e.g. a thin glass plate) is held a fixed distance from the Siwafer (e.g. 5-50 microns) and can be moved between regions containingmaterials, such as Sb, Al or no coating, so that the laser can eithertransfer materials (e.g. Sb or Al) or ablate a dielectric surface on theSi wafer. By designing the system with interchangeable optics, one couldlaser transfer and dope p⁺ and n⁺ point contacts and then switch to alow-power laser transfer of an interdigitated finger pattern that wouldlie on top of the dielectric passivation. The laser beam can be scannedacross the transparent transfer substrate and the silicon wafer to formthe desired contact pattern on the surface of the wafer. Theconductivity of the transferred contact pattern can be increased byplating the seed layers with a conductive metal such as Al, Ag or Cu.The laser transfer substrate may be first coated with a thin easilyevaporated material (e.g. a-Si:H) to act as a release layer for arefractory material (e.g. Mo) or a transparent material (e.g. SiO₂)deposited on the a-Si:H.

There are several advantages to laser-transfer processing. First, narrowline-shaped or very small diameter circular laser beams create lesslaser-induced damage than conventional Gaussian laser beams. Further,the processing throughput can be increased by about 100× with aline-shaped laser beam over that obtained with pulsed or Gaussian laserbeams. The processing throughput can be further increased by usingdiffractive optics or beam splitting optics in conjunction with apowerful laser to control the shape of the line-shaped laser beams orGaussian laser beams and/or to create multiple parallel line-shapedbeams or multiple small diameter Gaussian beams. The laser-transferredline contacts can be closely spaced (e.g. 100-300 microns) so thatlateral resistance in the IBC solar cells is minimized. Processing costscan be reduced since the laser processing system has a smaller factoryfootprint and uses far less energy than conventional diffusion furnaces.Dangerous chemicals such as POCl₃ and BBr₃ can be eliminated. Processingyields will increase since the laser-transfer process takes place atroom temperature with minimal wafer handling. Ongoing advances in lasertechnology should continue to increase the power of laser processingsystems and to decrease the cost.

Embodiments described herein are included to demonstrate particularaspects of the present disclosure. It should be appreciated by those ofskill in the art that the embodiments described herein merely representexemplary embodiments of the disclosure. Those of ordinary skill in theart should, in light of the present disclosure, appreciate that manychanges can be made in the specific embodiments described and stillobtain a like or similar result without departing from the spirit andscope of the present disclosure. From the foregoing description, one ofordinary skill in the art can easily ascertain the essentialcharacteristics of this disclosure, and without departing from thespirit and scope thereof, can make various changes and modifications toadapt the disclosure to various usages and conditions. The embodimentsdescribed hereinabove are meant to be illustrative only and should notbe taken as limiting of the scope of the disclosure.

What is claimed is:
 1. A method for forming interdigitated back contactsof a solar cell, the method comprising: depositing at least one dopantmaterial on a transfer substrate; positioning the transfer substrate apredetermined distance from a solar substrate, wherein the solarsubstrate provides at least one passivation layer on a rear surface; andactivating a laser, wherein the laser produces line-shaped laser beamsor Gaussian laser beams, and the laser disrupts the passivation layerand transfers the at least one dopant material to the solar substrate toform a finger pattern.
 2. The method of claim 1, wherein the line-shapedlaser beam has a width less than 20 microns or the Gaussian laser beamhas a diameter less than 20 microns.
 3. The method of claim 1, whereinthe laser activation step produces localized n⁺ and p⁺ point contacts inthe finger pattern.
 4. The method of claim 1, further comprising platinga conductive metal on top of the finger pattern on the solar substrate.5. The method of claim 1, wherein the transfer substrate furthercomprises a layer of conductive metal, and the method further comprisesactivating the laser to transfer the conductive metal from the transfersubstrate to the solar substrate, wherein the conductive metal isdeposited on top of the at least one dopant in the finger pattern. 6.The method of claim 1, wherein the solar substrate provides an emitterovercoated by the at least one passivation layer, and the laseractivation step ablates the at least one passivation layer and theemitter.
 7. The method of claim 6, wherein the emitter is a diffusedemitter, a tunnel oxide emitter, or an amorphous silicon heterojunctionemitter.
 8. The method of claim 1, wherein the line-shaped laser beamsor Gaussian laser beams are temporally shaped.
 9. The method of claim 8,wherein the laser is a scanning laser system and the line-shaped laserbeams or Gaussian laser beams are pulsed at predetermined times as thescanning laser system passes along the solar substrate to form thefinger pattern.
 10. The method of claim 1, wherein diffractive optics orbeam splitting optics are provided for the laser to control a shape ofthe line-shaped laser beams or Gaussian laser beams or to create aplurality of the line-shaped laser beams or Gaussian laser beams. 11.The method of claim 1, wherein line contacts of a conductive metal aredeposited on top of the at least one dopant in the finger pattern, andthe line contacts are spaced equal to or between 100-300 microns apart.12. A laser transfer system for transferring materials to a solarsubstrate for a back-contact solar cell, the system comprising: atransfer substrate coated with at least one material to be transferredto a solar substrate, wherein the solar substrate provides at least onepassivation layer on a rear surface; a spacer separating the transfersubstrate from the solar substrate by a predetermined distance; and ascanning laser that can produce line-shaped laser beams or Gaussianlaser beams, wherein the scanning laser disrupts the passivation layerand transfers the at least one dopant material to the solar substrate toform a finger pattern when activated.
 13. The system of claim 12,wherein the line-shaped laser beam has a width less than 20 microns orthe Gaussian laser beam has a diameter less than 20 microns.
 14. Thesystem of claim 12, wherein the laser activation step produces localizedn⁺ and p⁺ point contacts in the finger pattern.
 15. The system of claim12, wherein the transfer substrate further comprises a layer ofconductive metal, the scanning laser transfers the conductive metal fromthe transfer substrate to the solar substrate, and the conductive metalis deposited on top of the at least one dopant in the finger pattern.16. The system of claim 12, wherein the scanning laser ablates localizedregions of at least one passivation layer and an emitter of the solarcell.
 17. The system of claim 12, wherein the line-shaped laser beams orGaussian laser beams are temporally shaped.
 18. The system of claim 16,wherein the line-shaped laser beams or Gaussian laser beams are pulsedat predetermined times as the scanning laser system passes along thesolar substrate to form the finger pattern.
 19. The system of claim 12,wherein the scanning laser system produces the line-shaped laser beams,and the laser system is capable of processing about 1 solar substrateper second.
 20. The system of claim 12, further comprising: diffractiveoptics for the scanning laser system, wherein the diffractive opticsallow control of a shape of the line-shaped laser beams or Gaussianlaser beams or provide a plurality of the line-shaped laser beams orGaussian laser beams, or beam splitting optics for the scanning lasersystem, wherein the beam splitting optics to create a plurality of theline-shaped laser beams or multiple Gaussian laser beams.